586 RESPIRATION OF THE EMBRYO : ALLANTOIS. 



whilst circulating in the vascular area, is sufficient. In Fishes, 

 no further provision is made for this process ; since, by the 

 time it would be required, the egg is hatched ; the young 

 animal comes forth into the medium it is permanently to 

 inhabit, its own gills come into play, and the air contained in 

 the water can act directly upon the blood circulating in the 

 vascular area. But in the higher oviparous animals, whose 



development proceeds further 

 before they leave the egg, a 

 special provision is made for 

 this purpose. On the third day 

 of incubation, in the Fowl, a 

 bag termed the Allantois (fig. 

 322, i) begins to sprout (as it 

 were) from the lower end of the 

 body ; and gradually enlarges 

 (fig. 323), passing round the 

 y embryo, and beneath its en- 

 Fig. 323.-EMBKTO OP Fowz.Twith the vel P in g membranes, so as 



Allantois, a, over which ramify the almost Completely to inclose 



umbilical vessels, b ; at c is seen the + npi a 11T .fonp n f fhie 



indication of the place of the external lt; - 



ear ; d, e, f, rudiments of the cere- bag is plentifully Supplied 

 bellum, optic ganglia, and cerebrum -ii. ui j i r it 



respectively. with blood-vessels from the 



embryo ; and as one side of 



it lies in close proximity with the membrane of the shell, 

 it is very advantageously situated for receiving the in- 

 fluence of the air. It thus serves as the temporary respi- 

 ratory apparatus of the Chick, up to the time when it is pre- 

 paring to quit the egg. 1 There is reason to believe that the 

 bird then receives air into its lungs, from the air-space formerly 

 mentioned ( 755), which increases in size as the contents of 

 the egg diminish in bulk by the evaporation of their watery 

 part. By the increased vigour which it thus acquires, it is 

 enabled to perform the movements requisite for extricating 



1 If the respiration of the embryo be prevented by rendering the 

 shell impermeable to air, its development is completely checked. No 

 means of accomplishing this is so effectual, as smearing the shell with 

 oil or grease of any kind. Hence the effect of the well-known practice 

 of buttering the surface of the egg, in preventing the chick from being 

 reared ; and the same operation, if performed when the egg is quite 

 fresh, will preserve it for some time fit for eating, its decomposition 

 being prevented by the complete exclusion of the air. 



